linux-kernel - Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}

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Message-ID: <20190906000908.xpvkuhun7v6onp6w@yavin.dot.cyphar.com>
Date:   Fri, 6 Sep 2019 10:09:08 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers

On 2019-09-06, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Sep 06, 2019 at 09:00:03AM +1000, Aleksa Sarai wrote:
> > > > +			return -EFAULT;
> > > > +	}
> > > > +	/* Copy the interoperable parts of the struct. */
> > > > +	if (__copy_to_user(dst, src, size))
> > > > +		return -EFAULT;
> > > 
> > > Why not simply clear_user() and copy_to_user()?
> > 
> > I'm not sure I understand what you mean -- are you asking why we need to
> > do memchr_inv(src + size, 0, rest) earlier?
> 
> I'm asking why bother with __ and separate access_ok().

Ah right, it was a dumb "optimisation" (since we need to do access_ok()
anyway since we should early -EFAULT in that case). I've dropped the __
usages in my working copy.

> > > 	if ((unsigned long)addr & 1) {
> > > 		u8 v;
> > > 		if (get_user(v, (__u8 __user *)addr))
> > > 			return -EFAULT;
> > > 		if (v)
> > > 			return -E2BIG;
> > > 		addr++;
> > > 	}
> > > 	if ((unsigned long)addr & 2) {
> > > 		u16 v;
> > > 		if (get_user(v, (__u16 __user *)addr))
> > > 			return -EFAULT;
> > > 		if (v)
> > > 			return -E2BIG;
> > > 		addr +=2;
> > > 	}
> > > 	if ((unsigned long)addr & 4) {
> > > 		u32 v;
> > > 		if (get_user(v, (__u32 __user *)addr))
> > > 			return -EFAULT;
> > > 		if (v)
> > > 			return -E2BIG;
> > > 	}
> > > 	<read the rest like you currently do>
> 
> Actually, this is a dumb way to do it - page size on anything
> is going to be a multiple of 8, so you could just as well
> read 8 bytes from an address aligned down.  Then mask the
> bytes you don't want to check out and see if there's anything
> left.
> 
> You can have readability boundaries inside a page - it's either
> the entire page (let alone a single word) being readable, or
> it's EFAULT for all parts.
> 
> > > would be saner, and things like x86 could trivially add an
> > > asm variant - it's not hard.  Incidentally, memchr_inv() is
> > > an overkill in this case...
> > 
> > Why is memchr_inv() overkill?
> 
> Look at its implementation; you only care if there are
> non-zeroes, you don't give a damn where in the buffer
> the first one would be.  All you need is the same logics
> as in "from userland" case
> 	if (!count)
> 		return true;
> 	offset = (unsigned long)from & 7
> 	p = (u64 *)(from - offset);
> 	v = *p++;
> 	if (offset) {	// unaligned
> 		count += offset;
> 		v &= ~aligned_byte_mask(offset); // see strnlen_user.c
> 	}
> 	while (count > 8) {
> 		if (v)
> 			return false;
> 		v = *p++;
> 		count -= 8;
> 	}
> 	if (count != 8)
> 		v &= aligned_byte_mask(count);
> 	return v == 0;
> 
> All there is to it...

Alright, will do (for some reason I hadn't made the connection that
memchr_inv() is doing effectively the same word-by-word comparison but
also detecting where the first byte is).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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